Epoxy resins have been widely used in microeletrical industry as an encapsulating material to encapsulate semiconductor devices. The epoxy resins used as an encapsulating material mostly contain o-cresol novolac epoxy resin as a main ingredient. Thus the purity of the epoxy resin determines the purity of the encapsulating material prepared therefrom and also directly affects a reliance and a life of a semiconductor device encapsulated with the encapsulating material. The purity of the o-cresol novolac epoxy resin mainly depends on the content of halides and the presence of volatile substances therein. Generally, halides content above a certain level will cause a corrosion of wires on a wafer and then cause a damage of the wafer. Moreover, the presence of the volatile substances will cause a reduction of heat resistance of a solder and result in the release of the resin encapsulant from the wafer. Therefore, reducing the halides contents and removing of volatile substances are very important for an epoxy resin for encapsulating a semiconductor.
In a process for preparing a high purity epoxy resin, one or more organic solvents are used to aid the reaction. If such a process uses an alcohol as the solvent, and inter reaction between an active hydrogen on the alcohol and the epihalohydrin will occur to produce undesirable by-product. To avoid the production of the by-product, there are many patents disclosing the use of a solvent containing no active hydrogen, for example Japanese Patent Examined Publication No. 62-34330 and Japanese Patent Unexamined Publication No. 61-195111. However, the halide content of the epoxy resin prepared by the processes does not meet the requirement of high purity required by the microelectrical industry. Additionally, Japanese Patent Unexamined Publication No. 58-189223 disclosed a process for preparing an epoxy resin in high purity by using high-boiling solvent. However, the process will cause the contamination of the epoxy resin.
In view of this, the present inventors have widely investigated on the process for preparing epoxy resins and found that an epoxy resin with high purity and low halide content can be obtained by using specific solvents and thus completed this invention.